Electro-optical device and manufacturing method thereof

ABSTRACT

To suppress the occurrence of a failure caused by static electricity in the manufacturing process of an active matrix type display device in which an active matrix circuit and peripheral drive circuits are integrated on a glass substrate, a protective capacitor to be connected to a short ring is formed using a semiconductor layer made from the same material as the active layer of a thin film transistor present under the short ring. This protective capacitor has a function to absorb an electric pulse generated in the plasma using process. Discharge patterns are provided to prevent an electric pulse from affecting each circuit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a structure of an active matrix typeflat panel display device incorporating peripheral drive circuitstherein.

2. Description of the Related Art

Heretofore, there has been known an active matrix type liquid crystaldisplay device incorporating peripheral drive circuits therein. This hasa structure in which an active matrix circuit constituting pixel regionsformed of thin film transistors (abbreviated as a TFT) and peripheraldrive circuits for driving this active matrix circuit, which are alsoformed of thin film transistors, are integrated on a glass substrate (ora quartz substrate).

For example, in a VGA panel, about 300,000 thin film transistors areintegrated on the same glass substrate or quartz substrate. Also, in thecase of an EWS panel, about 1,300,000 thin film transistors areintegrated on the same glass substrate or quartz substrate.

In the above structure, if one of the thin film transistors isdefective, a dot defect or a linear defect is formed.

The performance of a display device is judged visually. Therefore, whenthe above dot defect or linear defect is present, the display device isjudged as a defective product.

When a glass substrate or a quartz substrate is used, especially, theproblem of breakdown by static electricity (electrostatic breakdown) isactualized because the insulating property of the substrate is high andits area is large.

For instance, in the formation of a liquid crystal panel, a plasma usingprocess is frequently used in the formation of various thin films andetching. In the plasma using process, pulse-form static electricity isgenerated as will be described hereinafter. Also, a process where staticelectricity is generated is existent such as a rubbing process otherthan the above plasma process.

As described above, in the formation of thin films constituting thinfilm transistors and etching, a plasma process typified by a plasma CVDmethod and a plasma etching method is frequently used. However, sincethe insulating property of a substrate used is high, there occurs such aphenomenon that discharge takes place locally in this plasma process.

A failure which is considered to be caused by this discharge occurs.Stated more specifically, the operation failure of a thin filmtransistor which is considered to be caused by various electrostaticbreakdowns or static electricity occurs. The failure is the major causeof a reduction in the production yield of an active matrix type liquidcrystal display device and other active matrix type flat panel displaydevices.

As the result of analyzing some examples of the occurrence of the abovefailures, the inventors of the present invention have reached thefollowing findings.

Firstly, there are roughly classified into two different types of theoccurrences of failures caused by static electricity or application oflocalized high voltage.

The first type of occurrences are caused by electrostatic pulses. Thefailures caused by the electrostatic pulses include a contact failureand the dielectric breakdown of an insulating film.

The contact failure is caused by the following mechanism. Firstly, atthe time of forming a thin film by a plasma CVD method or plasma etchingby an RIE method, localized discharge occurs. This discharge is causedby such a factor that a sample using an insulating substrate has a largearea and a state where localized discharge is liable to occur isestablished and further such minor factors as the uneven surface of apattern, the difference of pattern area, the slight difference of filmquality, the presence of particles and the like.

As the result of the above localized discharge, high voltage isinstantaneously applied to an extremely small specific region. At thispoint, voltage is locally induced in part of wiring and an electrostaticpulse is generated. This electrostatic pulse is generatedinstantaneously and a leading value of this induced voltage is extremelylarge.

A large current flows through a contact portion between a thin filmtransistor and wiring (or electrode) due to this electrostatic pulse.The instantaneous flow of a large current causes the contact portion togenerate heat at a high temperature. Thereby, the contact is broken. Thebreakdown of this contact is permanent and is generally difficult to berepaired.

Further, the breakdown of an insulating film is due to the fact that alocalized strong field is applied to the insulating film which mustretain an insulation function and the insulating property of thatapplied portion is broken by the instantaneous flow of a large currentcaused by electrostatic pulses through wiring and electrodes. Thebreakdown of this insulating property is also permanent and is generallydifficult to be repaired.

The second type of occurrences are caused by the generation of staticelectricity induced by plasma. This is caused by nonuniformity (such asarea difference or level difference) in the shape of a wiring pattern onthe substrate in the plasma using process such as film formation oretching. In this process, a localized potential difference isinstantaneously induced between patterns during plasma discharge.

This localized potential difference causes localized discharge betweenconductive patterns or between a conductive pattern and an insulatingsubstrate. This results in damage to a junction (such as a PI junctionor NI junction) of a thin film transistor, whereby the thin filmtransistor malfunctions.

The damage to the junction of the thin film transistor by this localizeddischarge may be repaired by a heat treatment. Therefore, the failure inthis case can be considered as semi-permanent.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a technology forimproving the production yield of a liquid crystal panel by suppressingthe occurrence of a failure caused by the above electrostatic breakdown.

The present invention is predicated upon the result of the aboveanalysis. The present invention has basically two aspects. According toa first aspect of the present invention, there is provided means forsuppressing the generation of electrostatic pulses. According to asecond aspect of the present invention, there is provided means forsuppressing the generation of static electricity induced by plasma.

In the present invention, to suppress the generation of electrostaticpulses, there are arranged protective capacitors for absorbinginstantaneous electric pulses around a liquid crystal panel.

In the manufacturing process of an active matrix type flat panel displaydevice typified by a liquid crystal panel, there is arranged wiringcalled “short ring” to eliminate a potential difference between wiringpatterns. This short ring is separated from a circuit in the end. In thestep of manufacturing a finished product, the short ring has no wiringfunction any longer.

One electrode of the above-described capacitor (protector capacitor) forabsorbing electric pulses is connected to this short ring. That is,electric pulses induced by this short ring are absorbed by the aboveprotective capacitor.

The short ring is connected to all source lines and gate linesconstituting an active matrix circuit. Therefore, even if an electricpulse enters somewhere in the active matrix circuit, it is absorbed bythe above protective capacitor. Even if the electric pulse is large andis not completely absorbed by the protective capacitor, its influencecan be weakened.

Generally speaking, the short ring is not connected to all the gateelectrodes of thin film transistors constituting peripheral drivecircuits for driving the active matrix circuit. However, when theprotective capacitor is arranged in an area near the peripheral drivecircuit block, it can absorb electric pulses from the outside andprevent the electric pulses from going into the peripheral drivecircuits. Further, the electric pulses which enter the peripheral drivecircuits can be weakened.

In the present invention, as means for preventing static electricityinduced by plasma, a discharge pattern for discharging this staticelectricity is arranged between the short ring and the active matrixcircuit region and between the short ring and the peripheral drivecircuit region.

According to the analysis of the inventors of the present invention,static electricity induced by plasma is easily generated mainly from aconductive pattern having a large area.

A conductive wiring pattern having the largest area in the manufacturingprocess of a liquid crystal panel is the short ring. That is, the shortring is used to eliminate a potential difference between wirings andsuppress unnecessary discharge. On the other hand, the short ring causesthe generation of static electricity by itself.

When voltage is induced by the short ring, static electricity is locallygenerated.

To cope with this, in the present invention, a discharge pattern (called“guard ring”) is provided between the short ring and the active matrixcircuit region and between the short ring and the peripheral drivecircuit region to cancel static electricity induced by plasma.

That is, before static electricity induced by the short ring enters theactive matrix circuit region or the peripheral drive circuit region,this static electricity is discharged in the step where it passesthrough the discharge pattern.

Alternatively, in a region which is affected by a potential difference,a discharge pattern is provided between the short ring and the circuit.Thus, the circuit is prevented from being affected by a potentialdifference which is produced between the circuit and the short ring.

Thus, it is possible to suppress damage to the thin film transistorsarranged in the active matrix circuit region and the peripheral drivecircuit region caused by static electricity induced by the short ring.

One aspect of the present invention, as shown by an embodiment thereofin FIG. 1, is a display device comprising an active matrix circuit 108and peripheral drive circuits 104 and 105 for driving the active matrixcircuit, arranged on the same substrate 101, characterized in that theactive matrix circuit 108 and peripheral drive circuits 104 and 105 aresurrounded by discharge patterns 112, 103 and 106.

In the above configuration, it is advantageous that the pitch of thedischarge patterns is smaller than the pitch of pixels of the activematrix circuit.

This is intended to prevent discharge from being produced in the activematrix circuit by an electric pulse which enters the active matrixcircuit.

Another aspect of the present invention, as shown by a manufacturingprocess according to an embodiment in FIG. 3, is an active matrix typedisplay device comprising an active matrix circuit (pixel region)arranged on the same substrate 301 and a capacitor formed adjacent tothe active matrix circuit, characterized in that the capacitor comprisesan electrode 307 formed in the same layer and from the same material asa gate electrode 310 of a thin film transistor arranged in the activematrix circuit, an insulating film 306 made from a material forming thegate insulating film of the thin film transistor under the electrode,and a semiconductor film 302 constituting the active layer of the thinfilm transistor under the insulating film 306.

Another aspect of the present invention, as shown by an embodiment inFIG. 3, is a method for manufacturing a display device comprising anactive matrix circuit (pixel region) and peripheral drive circuits fordriving the active matrix circuit, arranged on the same substrate 301,characterized by comprising the steps of:

forming a short ring 307 to be connected to all the gate lines and allthe source lines constituting the active matrix circuit;

forming impurity regions 319 to 321 and 300 for thin film transistorsarranged in the active matrix circuit by the implantation of impurityions; and

forming a capacitor in the short ring region by implanting impurity ionsinto a semiconductor layer 302 under the short ring using the short ring307 as a mask simultaneously with the above step.

In the above constitution, the capacitor is formed in a region where theelectrode (short ring) 307 and a semiconductor area 322 face each otherthrough the insulating film 306. This capacitor functions as aprotective capacitor for absorbing an electric pulse.

Another aspect of the present invention, as shown by an embodiment inFIG. 3, is a method for manufacturing an active matrix type displaydevice, characterized by comprising the steps of forming a thin filmsemiconductor layer 302 under the short ring 307, and forming acapacitor using the thin film semiconductor layer 302.

Another aspect of the present invention, as shown by an embodiment inFIG. 3, is a method for manufacturing an active matrix display device,characterized by comprising the step of forming a capacitor using a thinfilm semiconductor layer 302 which is present under the short ring 307simultaneously with the step of forming impurity regions 319 to 321 and300 for thin film transistors arranged in the active matrix circuit.

BRIEF DESCRIPTION OF THE INVENTION

These and other objects and advantages of the present invention willbecome apparent from the following description with reference to theaccompanying drawings, wherein:

FIG. 1 is a diagram showing an outline of an active matrix type liquidcrystal panel;

FIG. 2 is an enlarged view of an active matrix circuit and a short ringconnected thereto;

FIGS. 3A to 3D are sectional views showing a manufacturing process of anactive matrix type liquid crystal panel; and

FIG. 4 is a photograph of fine patterns formed on a substrate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the process of manufacturing an active matrix type liquid crystaldisplay device shown in FIG. 1, a short ring 102 is arranged to surroundan active matrix circuit 108 whose enlarged view is given in 100 andperipheral drive circuits 104 and 105. This short ring 102 is connectedto all source lines 110 and gate lines 111 arranged in a lattice formand constituting the active matrix circuit.

A MOS capacitor 107 made from a semiconductor used to constitute theactive layer of a thin film transistor is arranged using this short ring102 as one electrode thereof. This MOS capacitor 107 is a protectivecapacitor having a function to absorb a pulse voltage induced from theoutside.

A discharge pattern 112 is arranged between the short ring 102 and theactive matrix circuit 108. This discharge pattern 112 is also arrangedbetween peripheral drive circuits denoted by 104 and 105 and the activematrix circuit 108.

Discharge patterns 103 and 106 are further arranged between the shortring 102 and the peripheral drive circuits 104 and 105. These dischargepatterns 103 and 106 have a function to discharge a pulse voltageinduced by the short ring and suppress the entry of an electric pulseinto each circuit.

These discharge patterns 103 and 106 have a shape denoted by 203 in FIG.2, for instance. These discharge patterns are formed simultaneously withthe formation of the short ring and electrically interconnected asrequired.

It is effective to make the pitch of the discharge patterns shorter thanthe pitch of the wiring patterns of the circuits. This makes it possibleto previously discharge, in the discharge pattern, electric pulses,which are apt to be locally discharged in the circuits.

Embodiment 1

In this embodiment, the manufacturing process of a substrate on which anactive matrix circuit of an active matrix type liquid crystal displaydevice is formed using the present invention is outlined.

FIGS. 3A to 3D show the outline of the manufacturing process of anactive matrix substrate. FIGS. 3A to 3D show the process for forming Nchannel type thin film transistors arranged in pixel regions, P and Nchannel type thin film transistors arranged in peripheral circuitregions and a protective capacitor (capacitor for absorbing an electricpulse) arranged in a region where the short ring is formed on the samesubstrate simultaneously.

First, a silicon oxide film or silicon oxynitride film (not shown) isformed as an underlying film on a glass substrate 301 shown in FIG. 3A.As the substrate 301 may be used a quartz substrate.

After the underlying film (not shown) is formed, a silicon film isformed which constitutes the active layer of a thin film transistor anda capacitor in the later process.

In this embodiment, an amorphous silicon film is first formed by aplasma CVD method or a low pressure thermal CVD method. This amorphoussilicon film is further crystallized by a heat treatment and/or anirradiation of a laser light to obtain a crystalline silicon film (notshown).

This crystalline silicon film (not shown) is then patterned to formpatterns denoted by 302 to 305.

A pattern 302 is a semiconductor pattern constituting one electrode of aprotective capacitor formed on the short ring region.

Further, patterns 303 and 304 are semiconductor patterns constitutingthe active layers of a P channel type thin film transistor and an Nchannel type thin film transistor arranged in the peripheral drivecircuits. The pattern 303 serves as the active layer of the P channeltype thin film transistor and the pattern 304 serves as the active layerof the N channel type thin film transistor.

Further, a pattern 305 serves as the active layer of an N channel typethin film transistor arranged in the pixel region. The thin filmtransistor arranged in the pixel region is provided in each of pixelelectrodes arranged in a matrix form for switching.

Thus, a state shown in FIG. 3A is obtained. Thereafter, an insulatingfilm 306 constituting a gate insulating film and the dielectric of theprotective capacitor for absorbing an electric pulse in other regions isformed. In this embodiment, a silicon oxide film is formed to athickness of 1,000 Å by a plasma CVD method as the insulating film 306(FIG. 3B).

Thus, a state shown in FIG. 3B is obtained. Next, an aluminum film (notshown) constituting a gate electrode is formed by a sputtering method.While the gate electrode is formed of an aluminum film in thisembodiment, other metal materials, alloys and various silicide materialsmay be used.

This aluminum film contains 0.1% by weight of scandium. This is intendedto suppress the generation of a hillock or whisker caused by theabnormal growth of aluminum. The term “hillock” or “whisker” refers to aneedle- or thorn-shaped protrusion formed by the abnormal growth ofaluminum.

This aluminum film is then patterned to form aluminum patterns 307, 308,309 and 310 shown in FIG. 3C.

307 is a pattern constituting the short ring. That is, 307 shows thecross section of the short ring.

308 is a pattern constituting the gate electrode of a P channel typethin film transistor arranged in the peripheral drive circuit regions.309 is a pattern constituting the gate electrode of an N channel typethin film transistor arranged in the peripheral drive circuit region.

Further, 310 is a pattern constituting the gate electrode of an Nchannel type thin film transistor arranged in the pixel region. The gateelectrode 310 of the thin film transistor arranged in the pixel regionis formed such that it is extended from a gate wiring 202 which isarranged in a matrix form as shown in FIG. 2.

After the aluminum patterns 307 to 310 are formed, anodization iscarried out in an electrolyte solution using the patterns as anodes. Inthis step, anodized films 311, 312, 313 and 314 are formed.

In this anodization step, an ethylene glycol solution containingtartaric acid and neutralized with aqueous ammonia is used as theelectrolyte solution. The anodized films formed in this step are of finefilm quality and have a function to protect the surface of an aluminumfilm physically and electrically.

In other words, the anodized films have a physical function to suppressthe generation of a hillock or a whisker and an electric function toenhance insulating property from a region around an aluminum pattern.

Thus, a state shown in FIG. 3C is obtained. After this state isobtained, impurity ions are implanted.

While a region where the active layer 304 is formed is covered with aresist mask (not shown), P (phosphorus) ions are implanted by a plasmadoping method.

As a result, N type impurity regions 320, 319, 321 and 300 serving assource and drain regions are formed.

Further, regions 315 and 316 are also formed as N type impurity regions.At least one of the N type impurity regions 315 and 316 serves as anelectrode of a protective capacitor.

In other words, one electrode of the protective capacitor is a region307 and the other electrode is a region 315 or 316 or both regions. Theinsulating film 306 which serves as a gate insulating film in anotherregion functions as the dielectric of this protective capacitor. Thus,the protective capacitor which is a MOS capacitor is formed in aself-aligning manner simultaneously with the formation of an N type thinfilm transistor.

Thereafter, B (boron) ions are implanted by masking a P-ion implantedregion with a new resist mask. As a result, a source region 317 and adrain region 318 for a P channel type thin film transistor are formed ina self-aligning manner.

Thus, a state shown in FIG. 3C is obtained. After the implantation ofimpurity ions is completed, irradiation of a laser light is carried outto activate the region where the impurity ion is implanted.

Thereafter, a silicon oxide film, a laminate film consisting of asilicon nitride film and a silicon oxide film, or a laminate filmconsisting of these films and a resin film is formed as an interlayerinsulating film 322.

Further, a contact hole is formed in the drain region of a thin filmtransistor in the pixel region to form a pixel electrode 30 formed ofITO.

Contact holes are then formed again to form a source electrode 323 and adrain electrode 324 of a P channel thin film transistor of a peripheraldrive circuit region. At the same time, a source electrode 326 and adrain electrode 325 of an N channel thin film transistor of theperipheral drive circuit region are formed. A source electrode 327 of athin film transistor in the pixel region is also formed simultaneously.These electrodes are each formed of a laminate consisting of a titaniumfilm and an aluminum film.

It should be noted that the source electrode 327 is formed such that itis extended from a source wiring 201 shown in FIG. 2.

The structure of the pixel region shown in FIG. 3 is formed in each ofseveral million of pixels arranged in a matrix form simultaneously.

Embodiment 2

In this embodiment, a discharge pattern for discharging an electricpulse generated in the short ring induced by a localized potentialdifference during plasma discharge is described.

FIG. 2 shows part of one substrate side of an active matrix type liquidcrystal panel having source lines 201 and gate lines 202 arranged in alattice form. In the figure, there are shown thin film transistors 205and 206 arranged in a matrix form and liquid crystals 207 and 208 drivenby the outputs of these thin film transistors.

FIG. 2 shows the step where the thin film transistors are completed andwiring of each region is ended. In this step, the gate lines areconnected to the short ring 204. The source lines 201 are also connectedto another region (not shown) extended from the short ring 204.

In the constitution shown in FIG. 2, a discharge pattern as shown by 203is formed on wiring connecting the short ring 204 and the active matrixcircuit.

This discharge pattern 203 has a function to discharge an electric pulsecaused by a potential difference produced between the short ring 204 andthe circuit region at the time of film formation or etching usingplasma.

In order to enhance the effect of the discharge pattern 203, it iseffective to make the pitch of the discharge patterns smaller than thepitch of pixels of the active matrix circuit.

The discharge patterns 203 are arranged to surround the active matrixcircuit. In this embodiment, discharge patterns denoted by 203 arearranged on wiring connecting the active matrix circuit and the shortring.

However, it is not always necessary to connect the discharge patterns203 to wiring. For instance, it is effective to arrange dischargepatterns having such a shape as shown by 203 between circuits possibleto have a potential difference, between a circuit and wiring, andbetween conductive patterns possible to have a potential difference.

Even in this case, an electric pulse caused by a potential differenceproduced by some reason (generally by a plasma step) can be extinguishedby the presence of the discharge patterns.

FIG. 4 shows a photomicrograph of an TFT substrate for an active matrixtype liquid crystal display device having the discharge patterns. Thephotomicrograph of FIG. 4 shows fine patterns formed on a glasssubstrate.

FIG. 4 shows an active matrix circuit with lattice-form wiring formed inan upper left part thereof. This photograph also shows wiring extendingto the short ring (unshown in the photo) from the active matrix circuit.

Gate lines extend in a horizontal direction in the active matrix circuitshown in an upper left part of the photograph. What extend in a verticaldirection are source lines.

Further, FIG. 4 shows a state where discharge patterns for dischargingan electric pulse are arranged in a region extending to the outside ofthe active matrix circuit from the gate lines and the source lines.

Discharge patterns are arranged adjacent to a corner portion of theactive matrix circuit where an electric pulse enters with ease. Thedischarge patterns occupy more than ¼ the total area in a lower rightpart of the photo. The discharge patterns are not directly connected tothe active matrix circuit.

With this constitution, it is possible to prevent the active matrixcircuit from being damaged by an electric pulse caused by plasmadischarge.

Making use of the present invention, it is possible to suppress theoccurrence of a failure caused by electrostatic breakdown and improvethe production yield of a liquid crystal panel. This technology can beapplied not only to an active matrix type liquid crystal display devicebut also to other active matrix type flat panel display devices.

1. A method for manufacturing a display device comprising: forming afirst semiconductor layer in an active matrix circuit region over asubstrate; forming a second semiconductor layer in a driver circuitregion over the substrate; forming a third semiconductor layer over thesubstrate; forming an insulating film over the first through the thirdsemiconductor layers; and forming first through third electrodes overthe first through third semiconductor layers, respectively, wherein thethird electrode surrounds the active matrix region and the drivercircuit region, wherein the third semiconductor layer has a larger widththan that of the first and the second semiconductor layers, and whereinthe second semiconductor layer is located between the firstsemiconductor layer and the third semiconductor layer.
 2. A method formanufacturing a display device according to claim 1, wherein the firstthrough the third semiconductor layers are crystalline silicon layers.3. A method for manufacturing a display device according to claim 1,wherein the third semiconductor layer is an electrode for forming aprotective capacitor.
 4. A method for manufacturing a display deviceaccording to claim 1, wherein the display device is a liquid crystaldisplay device.
 5. A method for manufacturing a display device accordingto claim 1, wherein the first through the third semiconductor layers areformed simultaneously.
 6. A method for manufacturing a display devicecomprising: forming a first semiconductor layer in an active matrixcircuit region over a substrate; forming a second semiconductor layer ina driver circuit region over the substrate; forming a thirdsemiconductor layer over the substrate; forming an insulating film overthe first through the third semiconductor layers; forming first throughthird electrodes over the first through third semiconductor layers,respectively; and introducing an impurity element into portions of thefirst through the third semiconductor layers, wherein the thirdelectrode surrounds the active matrix region and the driver circuitregion, and wherein the second semiconductor layer is located betweenthe first semiconductor layer and the third semiconductor layer.
 7. Amethod of manufacturing a display device according to claim 6, whereinthe first through the third semiconductor layers are crystalline siliconlayers.
 8. A method for manufacturing a display device according toclaim 6, wherein the third semiconductor layer is an electrode forforming a protective capacitor.
 9. A method for manufacturing a displaydevice according to claim 6, wherein the display device is a liquidcrystal display device.
 10. A method for manufacturing a display deviceaccording to claim 6, wherein the first through the third semiconductorlayers are formed simultaneously.